In view of numerous factors such as higher energy prices and environmental concerns, the production of value-added products (such as pipeline-quality substitute natural gas, hydrogen, methanol, higher hydrocarbons, ammonia and electrical power) from lower-fuel-value carbonaceous feedstocks, such as petroleum coke, coal and biomass, is receiving renewed attention.
Such lower-fuel-value carbonaceous feedstocks can be gasified at elevated temperatures and pressures to produce a synthesis gas stream that can subsequently be converted to such value-added products.
One advantageous gasification process is hydromethanation, in which the carbonaceous feedstock is converted in the presence of a catalyst source and steam at moderately-elevated temperatures and pressures to directly produce a methane-rich synthesis gas stream (medium BTU synthesis gas stream) raw product. This is distinct from conventional gasification processes, such as those based on partial combustion/oxidation of a carbon source at highly-elevated temperatures and pressures, where a syngas (carbon monoxide+hydrogen) is the primary product (little or no methane is directly produced), which can then be further processed to produce methane (via catalytic methanation, see reaction (III) below) or any number of other higher hydrocarbon products.
Hydromethanation processes and the conversion/utilization of the resulting methane-rich synthesis gas stream to produce value-added products are disclosed, for example, in U.S. Pat. No. 3,828,474, U.S. Pat. No. 3,998,607, U.S. Pat. No. 4,057,512, U.S. Pat. No. 4,092,125, U.S. Pat. No. 4,094,650, U.S. Pat. No. 4,204,843, U.S. Pat. No. 4,468,231, U.S. Pat. No. 4,500,323, U.S. Pat. No. 4,541,841, U.S. Pat. No. 4,551,155, U.S. Pat. No. 4,558,027, U.S. Pat. No. 4,604,105, U.S. Pat. No. 4,617,027, U.S. Pat. No. 4,609,456, U.S. Pat. No. 5,017,282, U.S. Pat. No. 5,055,181, U.S. Pat. No. 6,187,465, U.S. Pat. No. 6,790,430, U.S. Pat. No. 6,894,183, U.S. Pat. No. 6,955,695, US2003/0167691A1, US2006/0265953A1, US2007/0000177A1, US2007/0083072A1, US2007/0277437A1, US2009/0048476A1, US2009/0090056A1, US2009/0090055A1, US2009/0165383A1, US2009/0166588A1, US2009/0165379A1, US2009/0170968A1, US2009/0165380A1, US2009/0165381A1, US2009/0165361A1, US2009/0165382A1, US2009/0169449A1, US2009/0169448A1, US2009/0165376A1, US2009/0165384A1, US2009/0217582A1, US2009/0220406A1, US2009/0217590A1, US2009/0217586A1, US2009/0217588A1, US2009/0218424A1, US2009/0217589A1, US2009/0217575A1, US2009/0229182A1, US2009/0217587A1, US2009/0246120A1, US2009/0259080A1, US2009/0260287A1, US2009/0324458A1, US2009/0324459A1, US2009/0324460A1, US2009/0324461A1, US2009/0324462A1, US2010/0071235A1, US2010/0071262A1, US2010/0121125A1, US2010/0120926A1, US2010/0179232A1, US2010/0168495A1, US2010/0168494A1, US2010/0292350A1, US2010/0287836A1, US2010/0287835A1, US2011/0031439A1 and GB1599932. See also Chiaramonte et al, “Upgrade Coke by Gasification”, Hydrocarbon Processing, September 1982, pp. 255-257; and Kalina et al, “Exxon Catalytic Coal Gasification Process Predevelopment Program, Final Report”, Exxon Research and Engineering Co., Baytown, Tex., FE236924, December 1978.
The hydromethanation of a carbon source typically involves four theoretically separate reactions:Steam carbon: C+H2O→CO+H2  (I)Water-gas shift: CO+H2O→H2+CO2  (II)CO Methanation: CO+3H2→CH4+H2O  (III)Hydro-gasification: 2H2+C→CH4  (IV)
In the hydromethanation reaction, the first three reactions (I-III) predominate to result in the following overall reaction:2C+2H2O→CH4+CO2  (V).
The overall hydromethanation reaction is essentially thermally balanced; however, due to process heat losses and other energy requirements (such as required for evaporation of moisture entering the reactor with the feedstock), some heat must be added to maintain the thermal balance.
The reactions are also essentially syngas (hydrogen and carbon monoxide) balanced (syngas is produced and consumed); therefore, as carbon monoxide and hydrogen are withdrawn with the product gases, carbon monoxide and hydrogen need to be added to the reaction as required to avoid a deficiency.
In order to maintain the net heat of reaction as close to neutral as possible (only slightly exothermic or endothermic), and maintain the syngas balance, a superheated gas stream of steam, carbon monoxide and hydrogen is often fed to the hydromethanation reactor. Frequently, the carbon monoxide and hydrogen streams are recycle streams separated from the product gas, and/or are provided by reforming/partially oxidating a portion of the product methane. See, for example, previously incorporated U.S. Pat. No. 4,094,650, U.S. Pat. No. 6,955,595 and US2007/083072A1. Required carbon monoxide, hydrogen and heat energy can also at least in part be generated in situ by feeding oxygen into the hydromethanation reactor. See, for example, US2010/0076235A1 and US2010/0287835A1.
The result is a “direct” methane-enriched raw product gas stream also containing substantial amounts of hydrogen, carbon monoxide and carbon dioxide which can, for example, be directly utilized as a medium BTU energy source, or can be processed to result in a variety of higher-value product streams such as pipeline-quality substitute natural gas, high-purity hydrogen, methanol, ammonia, higher hydrocarbons, carbon dioxide (for enhanced oil recovery and industrial uses) and electrical energy.
A solid char by-product stream is also produced, which contains unreacted carbon, entrained hydromethanation catalyst and other inorganic components of the carbonaceous feedstock. In hydromethanation processes, catalyst recovery from the char by-product and recycle of the recovered catalyst is typically present to improve economics and commercial viability. The nature of catalyst components associated with the char extracted from a hydromethanation reactor and methods for their recovery are disclosed, for example, in previously incorporated US2007/0277437A1, US2009/0165383A1, US2009/0165382A1, US2009/0169449A1 and US2009/0169448A1. Catalyst recycle can be supplemented with makeup catalyst as needed, such as disclosed in previously incorporated US2009/0165384A1.
The catalyst recovery and recycle process can be complicated based on the nature of the components present in the carbonaceous feedstock and, ultimately, the char by-product. For example, high-ash content carbonaceous feedstocks will bind more catalyst, requiring more intensive and complicated extraction processes to free the bound catalyst. In addition, metals and other components that may be extracted from the char with the catalyst can build up in the catalyst recycle stream, hydromethanation reactor and char by-product, necessitating the use of a bleed stream as part of the catalyst recycle, which results in additional catalyst losses and required catalyst makeup.
Certain carbonaceous feedstocks, such as those derived from petroleum-based materials (liquid petroleum resid, asphaltenes, petroleum coke and the like) may contain appreciable amounts of nickel and vanadium, which in and of themselves are valuable metals. The ability to recover vanadium is desirable, and processes for doing so in the context of a hydromethanation process are disclosed in U.S. Pat. No. 4,243,639 and US2011/0262323A1.
No mention, however, is made of recovery other valuable materials like nickel from the char. Therefore, a need remains for improved processes for hydromethanating vanadium-containing carbonaceous feedstocks to methane-enriched raw product gases, which processes integrate nickel recovery as well.